By Chris Webb
More than half a century since it became the world’s first nuclear power generator, the UK has signalled its endorsement of a technology that remains embroiled in controversy. Despite concerns of its critics about the perceived long-term hazards associated with nuclear waste, London’s earlier antipathy towards atomic power has turned to outright enthusiasm in what some interpret as a thinly disguised, salutary solution in the battle to reduce carbon emissions and slow global warming. But one nonetheless warmly welcomed by a lately moribund nuclear industry.
Currently undergoing a ‘generic design assessment’ (GDA), four new designs for a ‘third generation’ of nuclear reactors could set the UK on a course to generate more atomic power for up to 60 years. They represent a significant departure from Britain’s troublesome legacy of serial prototypes, which so damaged the industry’s fortunes in the last three decades, after the relatively successful and repeatable Magnox designs of the 1950s and ‘60s. Modular construction concepts mean that much of the building work is done off-site, raising the possibility of plants up-and-running in as little as three years from planning consent. It is a far cry from the dark days of AGRs such as Dungeness B, which took 17 years to complete.
The heat is on
The pace is hotting up. Last November, British Energy secured transmission connection agreements with National Grid from 2016 onwards for the four key sites it owns in the south of England: Sizewell in Suffolk; Hinkley Point in Somerset; Dungeness in Kent; and Bradwell in Essex. Subject to National Grid obtaining the necessary consents, this added connection capacity will accommodate any potential new nuclear power stations at these sites.
In May 2007, London invited applications from vendors of nuclear reactors interested in having their designs assessed, and in July it was announced that four applications had been made which met its criteria, which included having the support of a ‘credible’ nuclear power operator. These designs are: the Atomic Energy of Canada Limited ACR-1000; EDF/Areva European Pressurized Reactor (EPR); GE-Hitachi GEH ESBWR; and Toshiba Westinghouse AP1000.
The regulators believe the GDA process will enable them to get involved at the earliest stage in a project, long before a site-specific proposal is made. This in turn will allow better focusing of regulatory resource at a stage when greater influence can be brought to bear on the safety standards and features adopted in any proposed design. Moreover, and more important to investors, it is believed the process of separating design issues from site and operator issues will improve the efficiency of the regulatory process, reduce the risks for operators during the later licensing phase, and increase levels of certainty normally associated with the investment process.
The four designs share a number of common features. Aside from modular construction, making it easier to assemble many components elsewhere and in a controlled environment, ready to be shipped to the reactor site, all four manufacturers claim their designs are evolutionary in nature, a consequence arising from many years of experience operating reactors. This has allowed engineers to simplify designs and cut construction and generation costs, while improving safety measures. They stress that safety is of paramount importance, and incorporates the latest ‘passive’ features. In other words, in the event of a severe accident, safety systems use natural forces such as gravity, fluid circulation and evaporation, rather than ‘active’ systems which require many pumps, motors and valves whose freedom from failure cannot be guaranteed.
Waste, of course, is a key issue with which the industry continues to grapple. But industry experts say the new more efficient reactors, over their design lives, will generate only ten per cent of the waste produced by the UK’s entire nuclear sector to date.
Sky-rocketing costs that have beleaguered the industry in the past are another issue that designers have gone to lengths to address. Manufacturers say the final figure depends on a number of factors, such as location, the number of reactors at any one site, the number ultimately built offering economies of scale, and planning and licensing process issues, but each plant could be built for between £400 and £970 million ($800 million-$1.9 billion) at today’s prices. All four have a design life of 60 years.
Toshiba Westinghouse AP1000
While some detail other than that needed to support a GDA application remains a closely guarded commercial secret with the four’s designs, regulators will have their time cut out to plough through the labyrinthine documentation submitted by the four applicants. Especially that relating to safety. Westinghouse is keen to stress that the key issues of safety, cost and ease of construction have been built into the design for its AP1000, a pressurized water reactor, rated at 1117 MW has only half the number of valves typically found in earlier designs offering the same power. Currently the only one of the four designs to be licensed by the Nuclear Regulatory Commission (NRC) in the USA, a number of units have been ordered for China. Capable of leaving only a relatively small footprint on its chosen sites by comparison to its competitors, an AP1000 could be built in three years, or even less,
Regarded in some quarters as the favourite to win the race to be the first new third generation plant in the UK, the AP1000 ticks many of the regulators’ boxes. PWRs represent 76 per cent of all light water reactors in the world, and 67 per cent of those PWRs are based on Westinghouse PWR technology. Using proven technology, which builds on more than 35 years of operating PWRs, the AP1000 design includes advanced passive safety features and extensive plant simplifications to enhance the safety, construction, operation, and maintenance of the plant. While conservatively based on proven PWR technology, the AP1000 emphasizes safety features that rely on natural forces. Its safety systems use natural driving forces pressurized gas, gravity flow, natural circulation flow and convection to mitigate the risk of failure. It does not use active components (such as pumps, fans, or diesel generators), and is designed to function without safety-related support systems, such as AC power, component cooling water, service water, and heating, ventilation and air conditioning (HVAC). The number and complexity of operator actions required to control the safety systems are minimized; the approach is to eliminate operator action rather than automate it.
Westinghouse says that an important aspect of the AP1000 design philosophy focuses on plant operability and maintainability. The selection of proven components ensures a high degree of reliability with a low maintenance requirement. Component standardization reduces spare parts, minimizes maintenance and training requirements, and allows shorter maintenance durations. Built-in testing capability is provided for critical components.
The AP1000 design also incorporates radiation exposure reduction principles to keep worker dose as low as reasonably practicable (ALARP). Exposure length, distance, shielding, and source reduction are fundamental criteria incorporated into the design.
With the aim of minimizing construction time and total cost, individual component numbers, bulk quantities and building volumes are reduced. Some of these features include a flat, common nuclear island basemat design, which minimizes construction cost and programme.
In addition, integrated protection systems, advanced control room, distributed logic cabinets, multiplexing, and fibre optics significantly reduce the quantity of cables, cable trays, and conduits.
Westinghouse says the AP1000 safety philosophy of defence-in-depth enhances safety so that no large release of fission products is predicted to occur from an initially intact containment for more than 100 hours after the onset of core damage.
EDF/Areva UK EPR
EDF and Areva’s application for GDA is based on the Flamanville EPR (FA3) plant being built in France. The EPR features a four-loop, pressurized water reactor coolant system (RCS) composed of a reactor vessel that contains the fuel assemblies, a pressuriser including control systems to maintain system pressure, one reactor coolant pump (RCP) per loop, one steam generator (SG) per loop, associated piping, and related control and protection systems.
The RCS is sited within a concrete containment building. This is enclosed by a shield building with an annular space between the two. The pre-stressed concrete shell of the containment building has a steel liner. The containment and shield buildings comprise the reactor building. The reactor building is surrounded by four safeguard buildings and a fuel building. The internal structures and components within the reactor building, fuel building, and two safeguard buildings are protected against aircraft hazard and external explosions are separated by the reactor building, which restricts damage from these external events to a single safety division.
Redundant 100 per cent capacity safety systems (one per safeguard building) arranged in four trains is strictly separated into four divisions. This divisional separation is provided for electrical and mechanical safety systems. With four divisions, one of these can be out-of-service for maintenance and another can fail to operate, while the remaining two are available to perform the necessary safety functions even if one is ineffective because of an unscheduled event.
In the event of a loss of off-site power, each safeguard division is powered by a separate emergency diesel generator (EDG). In addition to the four safety-related diesels that power various safeguards, two independent diesel generators are available to power essential equipment during any station blackout (SBO) event for example the loss of off-site AC power resulting in coincident failure of all four EDGs.
Water storage for safety injection is provided by the in-containment refuelling water storage tank (IRWST). Also inside the containment, below the reactor pressure vessel (RPV), is a dedicated spreading area for molten core material following a postulated worst-case severe accident.
The fuel pool is located outside the reactor building in a dedicated building to simplify access for fuel handling during plant operation and handling of fuel casks.
AECL ACR-1000 PHWR
CANDU power plants an acronym derived from CANada Deuterium Uranium – a reference to its deuterium-oxide (heavy water) moderator and its use of uranium fuel, have been operating in Canada since 1962, in Asia since 1971, Latin America since 1983 and in Europe since 1996. The CANDU ACR-1000 is a light-water-cooled, heavy-water-moderated tube reactor and represents the latest design in a long line of reactors that stretches back to the 1940s, with strong connections to the UK through the Montreal project.
Schematic of a nuclear plant incorporating the CANDU ACR-1000 reactor design
Its horizontal fuel channels allow the reactor to be refuelled with slightly enriched uranium while it is still generating heat to power the steam generators, reducing the amount of time the plant is offline and therefore unavailable to supply electricity to the grid. It is expected that the first ACR1000 will be built in Canada, and producing electricity by 2014.
CANDU nuclear power plants are regarded as very safe. In almost 50 years of operation, no CANDU plant has ever had an incident of accidental release of radiation, and no member of the public has been harmed as a result of a radiation leak from a CANDU nuclear power plant or used fuel storage facility.
AECL says the many safety systems of the CANDU nuclear plant take into account human error, equipment failure, natural risks such as earthquakes, and even malicious acts such as terrorist attacks, which are a risk now being taken more seriously in nuclear power circles since the 9/11 catastrophe. The ACR-1000 incorporates CANDU safety features designed to prevent severe accidents and then to mitigate them by ensuring reactor shutdown, removing decay heat and preventing radioactive releases.
The inherent design of the ACR-1000, with its very small and negative void coefficient and the large volumes of cool heavy and light water in which the reactor core is immersed, combine to create a reactor that is inherently stable and readily controlled. AECL pioneered the use of advanced computer control systems in CANDUs in the 1970s and has applied this experience to the ACR-1000, which boasts one of the most advanced control and safety systems of any reactor.
In the event that an accident should occur, CANDU power plants have three means of shutdown, including the normal regulating system and two fast, independent, passively actuated safety shutdown systems that act within milliseconds in the event of abnormal conditions. CANDU is the only reactor design with two independent fast-acting safety shutdown systems.
The CANDU design is the only commercial reactor design whose core, made up of small diameter pressure tubes containing the nuclear fuel, is entirely immersed in a large tank of cool heavy water, which is itself immersed in an even larger steel lined concrete vault filled with cool water.
AECL says the presence of these large, passive heat sinks contributes an inherent safety advantage over pressure vessel reactors and slows the progression of potentially harmful events. These are supplemented by both passive and active systems designed to remove decay heat from the reactor.
The whole reactor and its primary systems are encased in a massive, steel-lined reactor building with steel reinforced concrete walls 1.8 m thick, designed to contain radioactive emissions and to withstand the direct impact of the largest commercial airliners. AECL pioneered the use of advanced computer control systems in CANDUs in the 1970s and has applied this experience to the ACR-1000.
The GEH ESBWR (Economic Simplified Boiling Water Reactor) has 25 per cent fewer pumps, valves and motors than the preceding reactor design, according to GE.
The principal plant structures include a reactor building housing all the safety-related structures, systems and components (SSCs), except for the main control room, safety-related distributed control and information system equipment rooms and spent fuel storage pool. This includes the reactor, containment, equipment rooms/compartments outside containment, the refuelling area with the fuel buffer pool, and auxiliary equipment area.
Once the poor relation, some would say historically, of the more sophisticated PWR, GE Hitachi Nuclear Energy’s (GEH) next evolution of advanced Boiling Water Reactor (BWR) technology is the ESBWR and brings the concept bang up to date. This simplified design provides improved safety, better plant security, a broad seismic design envelope, operational flexibility that increases plant availability and, crucially, says GE, better economics.
ESBWR is the latest in a long line of proven GEH BWR reactors. Like the other three being considered for GDA, it employs passive safety design features. It is a simplified reactor design, allowing faster construction and lower costs.
A GEH-designed Gen III+ reactor, ESBWR is currently in the US Design Certification process. The Design Control Document was docketed by the NRC in 2005, which along with Construction and Operating License (COL) submissions in 2007 will support the commercial operation of new ESBWRs by 2015.
GEH says it is ready to support utilities looking to build an ESBWR nuclear power plant, with a well established global supply chain. The plant is designed to produce electricity from a turbine generator unit using steam generated in the reactor.
Heat removal systems are provided with sufficient capacity and operational adequacy to remove heat generated in the reactor core for the full range of normal operational conditions and anticipated operational occurrences. Backup heat removal systems are provided to remove decay heat generated in the core under circumstances wherein the normal operational heat removal systems become inoperative. The capacity of such systems is adequate to prevent fuel cladding damage.
GE says that the reactor building is able to withstand earthquakes, hurricanes and tornados. Although the possibility of a large aircraft colliding with the structure is not part of the formal design specification, the manufacturers say the outer shell is robust enough to withstand such an event.
The GDA process is expected to take around three-and-a-half years, but by then the big decisions will be out of the way before site-specific applications get under way. Manufacturers such as Westinghouse, Areva, GE and AECL will be hoping that the process is a success, as will British Energy.